Silicon microphotonics has generated an increasing interest in recent years. Integrating optics and electronics on the same chip would allow enhancement of integrated circuit (IC) performance. Furthermore, telecommunications could benefit from the development of low cost solutions for high-speed optical links. The realization of active photonic devices, in particular high speed optical modulators integrated in silicon-on-insulator (SOI) waveguides, is essential for the development of silicon microphotonics/nanophotonics.
Although silicon does not in normal circumstances exhibit a linear electro-optic (Pockels) effect, other mechanisms are available for modulation, including thermo-optic and plasma dispersion effects. Aside from these, further interesting methods have been reported which include using strain to introduce a Pockels effect, forming SiGe/Ge quantum wells to take advantage of the quantum-confined stark effect, and bonding III-V materials to make use of their stronger electro-optic properties. The disadvantage of these approaches is the complex or non-CMOS compatible fabrication processes involved. The thermo-optic effect in silicon is relatively, very slow and therefore has no real use for high speed applications. The plasma dispersion effect on the other hand is much more promising with most of the recent successful high-speed silicon modulators being based upon this effect, whilst using carrier injection, depletion or accumulation to cause the required changes in free-carrier concentration.
The plasma dispersion effect uses changes in the free-carrier concentration to cause modulation of the light passing through the device. The free-carrier concentration may be changed by injecting carriers into the device, depleting carriers from a region of the device or by causing an accumulation of charge carriers in a region of the device. Carrier injection is typically carried out in a PIN diode structure with the optical waveguide passing though the intrinsic region. When the diode is forward biased, carriers pass into the intrinsic region causing a change in refractive index. Carrier depletion can be based upon a PN junction diode in the waveguide. Reverse biasing the diode causes carriers to be swept out of part or all of the waveguide region, again resulting in a change in refractive index. Carrier accumulation involves the use of an insulating layer between P and N diode regions that will, when biased, cause an accumulation of free carriers on the edges of the layer, much like a capacitor. Carrier depletion and accumulation, unlike carrier injection, are not limited by the relatively long minority carrier lifetime in silicon and consequently the fastest reported devices have utilised these mechanisms.
The figures of merit for classifying optical modulators are as follows:                Electro-optic bandwidth: this indicates the high-speed cut off frequency and can be used to predict data transmission rates in the absence of an eye diagram.        Data transmission rate: this indicates the rate at which data can be transmitted, with 5 Gb/s, 10 Gb/s or 40 Gb/s normally being targeted.        Dynamic extinction ratio: this gives the difference between the modulators on and off power levels at a specified data rate. A large extinction ratio will allow for longer transmission lengths before data restoration is required.        Optical insertion loss.        DC extinction ratio: this indicates the low speed difference in on and off power levels.        VπLπ efficiency: since devices produce phase modulation which is later converted to intensity modulation, this describes the voltage-length product required to produce a π radian phase shift.        Size.        Power efficiency.        
Other than these quantifiable factors, however, it is also important to consider the ease of fabrication and expected tolerances in device performance caused by slight variations inherent in the fabrication processes used, as these can have a direct effect on production cost and device yield. Existing devices have improved the data transmission rate and VπLπ efficiency, but are not always practical for mass production due to their complex structure.